A wave gear device commonly used as a reduction drive comprises a device housing, a rigid internally toothed gear, a flexible externally toothed gear, and a wave generator. The rigid internally toothed gear is fixed to the device housing. The flexible externally toothed gear is supported by the device housing in a rotatable state via a bearing such as a cross roller bearing.
A cup-type wave gear device having a cup-shaped flexible externally toothed gear is a known example of a wave gear device. A cup-shaped flexible externally toothed gear comprises a flexible cylindrical barrel part, a diaphragm extending inward from one end of the barrel part, and a rigid boss formed as a continuation of the internal peripheral edge of the diaphragm. External teeth are formed in the external peripheral surface portion of the cylindrical barrel part on the side having the open end. In a flexible externally toothed gear of this configuration, the rigid boss is fixed to the inner race of the bearing.
Known examples of a structure for fixing the rigid boss of the flexible externally toothed gear and the inner race of the bearing include fixing structures that rely on screws (Patent Documents 1 and 2), fixing structures that use screws and knock pins (Patent Documents 3 and 4), and fixing adhesive-supplementing structures to fixing structures that rely on a screw and knock pins (Patent Document 4).
A known example of a hollow rotating actuator is one configured from a hollow motor and a cup-shaped wave gear device. In such a hollow rotating actuator, a sleeve is disposed on the inner side of a hollow motor shaft, and oil is prevented from leaking from the wave gear device into the hollow part (Patent Documents 5 and 6).    Patent Document 1: JP-A 2002-31150    Patent Document 2: JP-A 2002-21866    Patent Document 3: JP-A 2001-336588    Patent Document 4: JP-A 2000-9191    Patent Document 5: JP-A 2006-144971    Patent Document 6: JP-A 2001-304382
In a conventional fixing structure in which a rigid boss of a flexible externally toothed gear and a bearing inner race are fixed by screws, a sufficient screwed amount must be ensured between the two components in order to achieve sufficient fixing strength. Because the axial length of the screw-fixed portion cannot be shortened to a predetermined value or less, it is an obstacle to shortening the axial length of the wave gear device and flattening the wave gear device. Also necessary are operations such as machining a male screw into the external peripheral surface of the rigid boss, machining a female screw into the internal peripheral surface of the bearing inner race, and assembling the two components by screwing them together. This causes the problem of numerous component machining steps and assembling steps.